1. Field of the Invention
The invention relates to a side emitting glass element which laterally emits portions of the light guided in it, a method for producing said glass element, and the use thereof. Such side emitting glass elements are required for example for lighting applications, be it decorative lighting, signal lighting or the illumination of spaces.
2. Description of Related Art
The light guiding effect of light guided in glass elements is based on the principle of total reflection of the guided light at a medium which surrounds the glass element and which has a lower refractive index. Total reflection occurs when the surrounding medium has a lower refractive index than the glass element guiding the light. However, the condition of total reflection is possible only up to a critical angle of the light impinging on the medium, said angle being dependent on the refractive indices of glass element and surrounding medium. The critical angle βMin, i.e. the smallest angle at which total reflection still occurs, can be calculated by sin(βMin)=n2/n1, where βMin is measured from a plane perpendicular to the axis of the main direction of light guiding, n1 represents the refractive index of the glass element and n2 represents the refractive index of the medium. The surrounding medium can usually be either air, but also a glass cladding.
It is usually endeavoured to guide the light as well as possible in a light guiding element, that is to say that the intention is to lose as little light as possible during coupling into the element and during transport in the element. In a side emitting glass element, however, light is intentionally coupled out from the glass element and out of the latter. In general, a uniform coupling-out is desirable which, in the ideal case, causes a side emitting glass element to appear as a uniformly luminous strip or line. This makes it of interest for diverse applications particularly in lighting technology.
Side emitting in the sense of the invention means that the glass element is able to guide light but also to emit light laterally, independently of whether it is in operation, that is to say whether a light source is actually connected and the light is switched on.
Various methods for producing the effect of side emission in light guides are known from the prior art particularly with regard to fibres. One known method is to provide for a coupling-out of light in the fibre core.
The published Japanese patent application JP 9258028 A2 discloses side emitting stepped-index fibres in which the coupling-out of light is intended to be produced by means of a non-round core. The coupling-out is affected if light impinges on the interface between fibre core and cladding at angles which are less than the critical angle of total reflection βMin. As a result of the non-round core geometries described, for example, square, triangular or star shapes, geometrical regions in which light that is otherwise guided by total reflection can be coupled out are produced in the core. The production of side emitting fibres by means of such core geometries is beset by the problem, however, that the coupling-out of the light is very inefficient in this case. The light is guided in the fibre substantially at very shallow angles of incidence with respect to the cladding, and the core geometries described extend along the fibre axis. Accordingly, there are hardly any areas at which βMin is undershot. Furthermore, it is very complicated to use the core geometries disclosed in JP 9258028 A2 for fibres composed of glass, because it is very difficult to produce corresponding preforms such as are required for fibre drawing. Furthermore, precisely in the case of glass fibres, the breaking strength of such fibres having non-round fibre core diameters is greatly reduced. It is probably for this reason that said document also only discloses fibres composed of polymers.
A further method for coupling out the light from a fibre core is described in U.S. Pat. No. 4,466,697. Accordingly, particles that reflect and/or scatter light are mixed into the otherwise homogeneous fibre core. In this case, it proves to be difficult to produce relatively long fibres having uniformly side emitting properties since the light guiding in the core is attenuated by the admixed particles in the core by absorption since there are no particles which effect total scattering, but rather only those which only almost scatter the entire impinging light. Since, with particles distributed uniformly in the core, the probability is very high that the light guided in the core will impinge on such particles, the probability of absorption is also very high, even if the total number of particles is small. This means that the coupling-out effect can also be scaled only with very great difficulty, which makes reproducible results complicated to almost impossible.
Scalability in the sense of the present disclosure is understood to mean the possibility of the targeted setting of the side emission effect over the length of the glass element. This is necessary because the lengths thereof can vary very greatly for different applications, but the intention is to achieve a light emission intensity that is as uniform as possible over the entire length of the element.
As an alternative to coupling out the light directly from the fibre core, side emitting properties in fibres can also be caused by effects in the interface between fibre core and cladding. WO 2009/100834 A1 provides for introducing scattering regions in the interface between core and cladding of a fibre. For this purpose, a corresponding material is melted onto the fibre core during fibre drawing. As a result of the contact between the scattering regions and the fibre cladding, a very efficient lateral coupling-out of light is effected, and the scattering regions themselves are protected by the surrounding material and are an integral part of the fibre. The side emitting light guiding fibres in the document just cited make it possible to produce flexible and relatively long side emitting light guides. However, experiments have revealed that if rigid light guides are produced with this solution, they usually have thousands of laterally emitting individual fibres which are mutually superimposed on one another and thus reduce the efficiency of the lateral emission and only a side emission profile extending radially along the fibre axis can be achieved. Rigid fibres that emit laterally into specific defined solid angles therefore cannot be produced or can at most be produced in an unfavourable manner. Moreover, a disadvantageous colour bleed effect can be observed, which occurs when white light is radiated into such rigid fibres. Said effect is presumably caused by the very small dimensions of the scattering centres used.
Rigid glass elements that emit laterally into defined solid angles are disclosed by DE 10 2011 084 062 A1, in the case of which glass elements a glass rod is provided with a colour coating on its outer circumference, e.g. by printing. This solution has the disadvantage that the coated glass rod can no longer be subjected to hot processing, that is to say that said rod can no longer be brought to a different form after coating. Moreover, the layer is sensitive. Alternatively, the glass rod may be coated after it has been subjected to hot forming, but this places higher demands on the coating technology and thus increases the production outlay in an unacceptable manner for many applications.